A backlight is typically used to illuminate an LCD panel for information display. A typical LCD, such as an active matrix LCD, is comprised of liquid crystal disposed between two substrates. Adjacent to one substrate is a thin film common electrode and adjacent to the other is an array of thin film electrodes. Each electrode in the array is connected to one or more thin active elements and address lines, which serve to regulate the voltage applied to the electrode. The liquid crystal disposed between each pixel electrode and the common electrode comprises a pixel of the LCD. The electrode of the array for a given pixel is referred to below as a “pixel electrode”.
In general, liquid crystal of a pixel of an LCD will pass incident light because of a helical alignment of the liquid crystal molecules working in conjunction with other optical elements. When a voltage is applied between a pixel electrode and the common electrode, for example, via the address lines and the active elements, the helical alignment of the liquid crystal is removed or modified and light is impeded or prevented from passing through the pixel.
The liquid crystal itself does not generate light, but instead either passes or impedes the passage of light as described above. Thus, a “backlight” is typically provided for the LCD, which provides light to the pixels of the LCD. Since the backlight is typically a white light source, for a color LCD each “pixel” is generally comprised of three separate pixels or “sub-pixels” covered with a red, green and blue filter. The color displayed by each pixel is controlled by the light emitted by each of the three sub-pixels, which is a function of the voltage between the pixel electrode (or, the “sub-pixel electrode”) for each sub-pixel.
The backlight provided to the LCD is, as noted, typically a white light source. It is generally desirable to have as constant an intensity as possible across the LCD panel to present a highly uniform image on the display. FIG. 1 depicts a basic design of a backlight 10 that uses a parabolic reflector 12 with a fluorescent light 14 positioned at the focal point of the reflector 12. The light rays from the light are thus reflected by the reflector 12 normal to a diffusion surface 16, which provides a more uniform distribution of light intensity emitted by the surface 16.
The backlight assembly 10 of FIG. 1 is impractical for some applications of an LCD panel because of the width required by the parabolic reflector 12. Referring to FIG. 2, a basic backlighting arrangement of an LCD panel 42 of a low-profile PC 40 is shown. The backlight of the LCD panel 42 is provided by two thin fluorescent lights 44a and 44b that reside at the edges of the LCD panel 42 within the thin display portion 46 of the PC 40. Light foils and guides (not shown) as known in the art serve to distribute the light from the fluorescent lights 44a, 44b that enters the edge of the LCD panel 42 so that it is relatively uniformly distributed among the pixels of the LCD panel 42 and is directed toward the viewing surface of the LCD panel 42 shown in FIG. 2.
The state of the art usage of fluorescent lighting as a backlight is based on cold cathode fluorescent lamp (CCFL) technology with high frequency (HF) inverter circuits. A limitation that arises, however, is that these backlighting systems typically produce white light having a pre-set color point. The relative amounts of light output of various color components (for example, red, green and blue) by the CCFL cannot be regulated by means of the drive electronics or in any other way it is generated. (The relative amounts of light output of various colors, in particular, the relative amounts of red, green and blue light output, will be referred to below as the “color content”. It is also related to the color point of the output in the backlight.) The CCFL backlight has limited potential in providing even rudimentary support of a video output, such as providing a variable color for a displayed image.
In addition, the CCFL would offer little potential for assistance in displaying a high quality image, such as a video feed with moving objects, on an LCD display. A typical response time of present LCDs (including drive circuitry) is on the order of 50 ms, whereas a video frame rate is on the order of 120 Hz, or a frame period on the order of 8.3 ms. Since the frame period is much less than the response time of the LCDs, the LCD panel suffers from certain artifacts in displaying fast moving video images. One way to reduce or remove the artifacts is to switch the LCD backlight on and off over a time interval that is a fraction of the frame period. However, present high frequency inverters and CCFLs have an on/off response time greater than 5 ms. Thus, the CCFL cannot be switched fast enough to eliminate the artifacts from the video displayed on the LCD display. Thus, CCFL technology offers limited potential as a high quality LCD panel backlight.